1. Field of the Invention
An electrochemical method and apparatus for the synthesis of nitrogen fertilizers including ammonium nitrate, urea, ammonia, and urea-ammonium nitrate is described herein. In particular, an apparatus and method is described whereby (1) a nitrogen source is utilized to produce ammonium nitrate; (2) a nitrogen source and a carbon source are reacted using liquid electrolyte at low temperature or solid electrolyte at high temperature to form urea; (3) a nitrogen source and a hydrogen-equivalent source are reacted to generate ammonia; and (4) a nitrogen source and carbon source are reacted to produce urea-ammonium nitrate.
2. Background of the Invention
Ammonium nitrate (AN, 34% N), urea (46% N), ammonia (82% N) and urea-ammonium nitrate (UAN, 28%˜32% N) are widely used high nitrogen-content fertilizers. Methods for industrial production of these fertilizers are mainly based on the Haber process, which involves the heterogeneous reaction of nitrogen and hydrogen on an iron-based catalyst at high pressure (for example, 200-300 bar) and high temperature (for example, 430° C.-480° C.) to produce ammonia as follows:N2 (g)+3H2 (g)2NH3 (g)  (Rea. 1)
The conversion to ammonia, shown in Reaction 1, is limited by thermodynamics. The gas volume decreases as the reaction progresses. Hence, very high pressure must be used to drive the ammonia synthesis reaction to the right in Reaction 1, which is in the direction of ammonia gas. Carrying out ammonia synthesis at very high pressure is also necessary to prevent decomposition of synthesized ammonia into nitrogen and hydrogen and to provide practical reaction rates. In addition, Reaction 1 is exothermic, and ammonia formation increases with decreasing temperature. Reducing the temperature, however, undesirably reduces the rate of the reaction. Therefore, an intermediate temperature is selected such that the reaction proceeds at a reasonable rate, but the temperature is not so high as to drive the reverse reaction. The equilibrium conversion of hydrogen gas and nitrogen gas to ammonia is generally only on the order of 10%˜15%. Low conversion efficiencies give rise to cost-intensive, large-scale chemical plants and costly operating conditions required to commercially produce hundreds to thousands of tons per day of ammonia in an ammonia synthesis plant.
Ammonium nitrate (AN) is produced via the acid-base reaction of ammonia with nitric acid according to the equation:NH3+HNO3→NH4NO3  (Rea. 2)
Industrial nitric acid is manufactured by the high-temperature catalytic oxidation of ammonia. This process typically consists of three steps: first, ammonia is reacted with air on PtIr alloy catalyst at around 750°˜800° C. to form nitric oxide according to the following reaction:4NH3+5O2→4NO+6H2O  (Rea. 3)Next, nitric oxide is oxidized to nitrogen dioxide and its liquid dimer as follows:2NO+O2→2NO2N2O4.  (Rea 4)And, finally, the nitrogen dioxide/dimer mixture is introduced into an absorption process using water in accordance with the following reaction:3NO2+H2O→2HNO3+NO  (Rea. 5)In the first step, the oxidation of ammonia to nitric oxide proceeds in an exothermic reaction with a range of 93% to 98% yield. Reaction temperatures can vary from 750° C. to 900° C. Higher temperatures increase reaction selectivity toward NO production. Reaction 3 is favored by low pressures. In the second step, Reaction 4 is slow and highly temperature- and pressure-dependent. Operating at low temperatures and high pressures promotes maximum production of NO2 within a minimum reaction time. The final step, Reaction 5, is exothermic, and continuous cooling is therefore required within the absorber. As the conversion of NO to NO2 is favored by low temperature, this reaction will take place significantly until the gases leave the adsorption column.
The commercial production of urea is based on the reaction of carbon dioxide and ammonia at high pressure (for example 140 bar) and high temperature (for example 180°˜185° C.) to form ammonium carbamate (Reaction 6), which is subsequently dehydrated into urea and water (Reaction 7):2NH3+CO2→NH2COONH4  (Rea. 6)NH2COONH4→NH2CONH2+H2O  (Rea. 7)
Reaction 6 is fast and highly exothermic and goes essentially to completion under normal processing conditions, while Reaction 7 is slow and endothermic and usually does not reach thermodynamic equilibrium under processing conditions. The degree to which Reaction 7 proceeds depends on, among other factors, the temperature and the amount of excess ammonia used. Increasing temperature and the NH3:CO2 ratio could increase the conversion of CO2 to urea.
Different urea production technologies basically differ on how urea is separated from the reactants and how ammonia and carbon dioxide are cycled. Refinements in the production technology are usually concentrated on increasing CO2 conversion, optimizing heat recovery, reducing utility consumption, and recovering residual NH3 and urea from plant effluents.
Ammonium nitrate and urea are used as feedstocks in the production of urea-ammonium nitrate (UAN) liquid fertilizers. Most UAN solutions typically contain 28%, 30% or 32% N, but other customized concentrations (including additional nutrients) are produced. The addition of corrosion inhibitors or the use of corrosion-resistant coatings allows carbon steel to be used for storage and transportation equipment for the solutions.
Continuous and batch-type processes are used, and, in both processes, concentrated urea and ammonium nitrate solutions are measured, mixed, and then cooled. In the continuous process, the ingredients of the UAN solution are continuously fed to and mixed in a series of appropriately sized static mixers. Raw material flow as well as finished product flow, pH, and density are continuously measured and adjusted. The finished product is cooled and transferred to a storage tank for distribution. In the batch process, the raw materials are sequentially fed to a mixing vessel fitted with an agitator and mounted on load cells. The dissolving of the solid raw material(s) can be enhanced by recirculation and heat exchange as required. The pH of the UAN product is adjusted prior to the addition of the corrosion inhibitor.
As described above, the production of high-nitrogen fertilizers involves multi step reactions and is strongly limited by the Haber process. The equilibrium conversion of hydrogen gas and nitrogen gas to ammonia in the Haber process is generally only on the order of 10%-15%. Such low conversion efficiencies give rise to cost-intensive, large-scale chemical plants and costly operating conditions required to commercially produce hundreds to thousands of tons per day of ammonia in an ammonia synthesis plant.
Recently, attention has been drawn to the removal of CO2 and nitrogen oxides from the environment, as it is conjectured that these compounds contribute to serious problems, including the “greenhouse effect” and acid rain.